1. Field of the Invention
The present invention relates to a process for producing an optical material such as a plastic lens, a prism, an optical fiber, a substrate of information recording materials and a filter, and more particularly, to a process for producing a plastic lens of glasses having excellent color tone and transparency.
2. Description of the Related Arts
Plastic materials have widely been used as various optical materials, particularly as lenses of their glasses, because of light weight, toughness and easiness of tinting. The properties required for optical materials, as particularly for lenses of glasses, are a low specific gravity and optical properties such as a large refractive index and a large Abbe number. It is also required that optical materials are colorless and transparent and show no change in color tone after use for a long time. A large refractive index is important to decrease the thickness of a lens. A large Abbe number is important to decrease the chromatic aberration of a lens and to decrease fatigue of eyes.
As conventional materials having a large refractive index, thermosetting optical materials having a thiourethane structure which are obtained by the reaction of a polythiol compound and a polyisocyanate compound have been proposed (Japanese Patent Publication Heisei 4(1992)-58489 and Japanese Patent Application Laid-Open No. Heisei 5(1993)-148340). Technology to obtain a lens by copolymerization of an epoxy resin or an epithio resin with a multi-functional compound have also been proposed in the specifications of Japanese Patent Application Laid-Open No. Heisei 1(1989)-98615, Japanese Patent Application Laid-Open No. Heisei 3(1991)-81320 and International Publication No. WO8910575. However, in accordance with the above technologies, a molecular design for an increase in the refractivity index causes a decrease in the Abbe number and it is difficult to obtain a balance between a sufficiently high refractivity index and a large Abbe number.
To overcome the above problem, the present inventors discovered compounds containing sulfur which have a novel structure containing an epithio structure or an epithioalkylthio structure and optical materials obtained by curing the compounds by polymerization.
However, the above optical materials tend to show change in color to a yellowish or greenish color when the materials are left standing for a long time and improvement in light resistance has been desired.
As the result of studies on the coloration, it was found that the problem can be overcome by addition of an ultraviolet light absorbent.
However, the addition of an ultraviolet light absorbent gave rise to a new problem in that the optical materials are colored after being prepared and are also clouded. In other words, the optical material having an epithio structure or an epithioalkylthio structure shows more marked coloring and clouding than those of conventional optical materials when an ultraviolet light absorbent is added. Therefore, it has been strongly desired that the above problems are overcome.